One method commonly employed to produce high density polyethylene (HDPE) creates an active site for polymerization by reacting an active catalyst with an ethylene monomer. Active catalysts for ethylene polymerization are known in the art as but not limited to Zeigler-Natta, metallocene, and chromium based catalysts. In particular, chromium based catalytic systems are typically chromium oxide on a high surface area silica, titania, and/or alumina support. To form the active catalyst from the chromium procatalyst, ethylene (or an alphaolefin) reduces the valence state of the chromium in the reaction medium. After the reduction process, presumably a Cr—C bond forms the active site for polymerization. Once the active site forms, the reaction mechanism is considered a transition metal catalyzed coordination polymerization.
The polymerization can take place in solution, slurry or gas phase. Preferably, the reaction takes place in a loop reactor where ethylene and an α-olefin comonomer (if used) circulate in the liquid phase. The catalyst and an inert solvent are introduced into the loop reactor, which is maintained at a temperature below the melting point of HDPE (about 135° C.) to ensure the polymer is formed in the solid state. The inert solvent and a cooling jacket are used to dissipate heat created by the highly exothermic reaction. The active sites on the catalyst are equally accessible to the ethylene throughout the catalyst particle. As such, the polymer chain grows not only outwards but also inwards, causing the granule to expand progressively. The resulting slurry of HDPE and catalyst particles circulates through the loop at a relatively high velocity to prevent the slurry from depositing on the walls of the reactor. The ethylene, α-olefin comonomer (if used), catalyst, and inert solvent are continuously charged into the reactor at a total pressure of, e.g., 450 psig. The slurry containing the polymer is continuously removed from the reactor. The solvent is recovered by hot flashing, and the polymer is dried before it is subjected to further processing, e.g., extrusion into pellets. The molecular weight of the HDPE can be controlled by the temperature of the chromium catalyst preparation, the temperature of the reactor, and by the addition of hydrogen into the reactor.
Another type of process used to create HDPE is gas-phase polymerization. In this process, an α-olefin is reacted with an active catalyst, typically a chromium based catalyst supported by silica, to form HDPE. Likewise, the molecular weight of the HDPE can be controlled by the temperature of the chromium catalyst preparation, the temperature of the reactor and by the addition of hydrogen into the reactor.
Polyolefins undergo oxidation when exposed to elevated temperatures in the presence of ambient oxygen. Polyolefins are exposed to high temperatures during melt processing operations, such as extrusion and injection molding. They can further be exposed to high temperatures during the course of their use. For example, a plastic bag made of polyethylene could be placed in a vehicle that is parked in the sun on a hot day. Thermal oxidation adversely affects the physical properties of the polyolefins. Phenolic antioxidants are commonly added to polyolefins to extend their long-term thermal stability. They are also effective in stabilizing polyolefins at the high temperatures encountered in melt processing operations.
Adding phenolic antioxidants to polyethylene produced using a chromium-based catalyst undesirably causes color bodies to form in the polyethylene. While not affecting the performance of the polyethylene, the presence of the color bodies gives the polyethylene an unappealing yellow appearance. While not wanting to be bound by theory, it is believed that upon exposure to high temperatures, unconsumed catalyst remaining in the polyethylene product oxidizes the phenolic compounds to form “quinones or similar structures” that are yellow in color. The concentration of the color bodies (believed to be quinines or similar structures) likely increases over time, and the process is accelerated as the polyethylene is repeatedly subjected to high temperature conditions. Consequently, the appearance of the polyethylene becomes more yellow as the polyethylene ages. Therefore, a need exists to inhibit the formation of color bodies in polyolefins that are produced using a chromium-based catalyst. The polymer composition of the present invention, which is resistant to color body formation, is less likely to turn yellow over time.